Automotive emissions have a negative impact on the Earth's environment. In an effort to reduce the effect of automotive emissions on the environment, extensive legislation has been passed to regulate the amount and types of emissions allowed for automobiles. As time has progressed, government regulations regarding the amount and types of emissions allowed has continued to become more strict. As a result, the automotive industry has continually explored technology to assist in emissions reduction. One possible technology being explored is hybrid or completely electric vehicles. These electric vehicles require the use of an electric motor as a power source.
Industrial applications such as electric vehicles, robotics, automated machine tools and precise positioning systems are generally implemented with DC-servo motors as power sources. DC motors are commonly used for applications requiring characteristics such as fast response and good low speed control because the torque developed by a DC motor is proportional to the armature current. The armature current of a DC motor is easily controlled using current feedback. Although the AC induction motor enjoys a number of advantages such as higher output per unit mass over the DC motor, the fact that rotor flux is induced rather than directly controlled has traditionally disqualified the AC induction motor from many high performance, high precision industrial applications.
The recent development of field-oriented control has now made fast torque control of induction motors achievable. Analogous to the DC machine, the torque control of an AC motor will be achieved through current control, however, in the AC case this must also include the phase as well as the amplitude.
In a DC motor the commutator and the brushes fix the orientation of the field flux and the armature mmf. However, in an AC machine, the orientation of the field flux and armature mmf must be controlled externally. If this orientation is not controlled in the AC motor then the angles between the various fields vary with loads and transients making for an oscillatory dynamic response. With the orientation between the fields controlled externally in the AC motor, current feedback can be applied to allow torque control.
Various approaches have been made to qualify the induction motor for high performance applications using various concepts of field oriented control. One approach involves the measurement of rotor flux and the determination of stator terminal excitation values needed to produce desired torque or speed conditions.
Also, overall drive system efficiency has not been an important issue until the advent of the electric vehicle. This is particularly important on electric vehicles because the range and acceleration of an electric vehicle is limited by its battery capacity.
The disadvantages associated with conventional motor control have made it apparent that a new technique for high efficiency motor control is needed. Preferably, the new technique should maximize motor torque and efficiency. The present invention is directed to these ends.